Electrical filtering apparatus



Se t. 5, 1933. L. D. WHlTEL-OCK ELECTRICAL FILTERING APPARATUS Filed Oct. 17. 1951 3 Sheets-Sheet l INVENTORY L,D, 1 /7/ te/oc/ BY 6m 242M M ATTORNEY Sept 5, 1933. L. D. WHITELOCK ELECTRICAL FILTERING APPARATUS Filed Oct. 17, 1931 3 Sheets-Sheet 2 INVENTOR,

Y mm m WM mmw Sept. 5, 1933. wl-HTELOCK 1,925,160

ELECTRICAL FILTERING APPARATUS Filed Oct. 17, 1931 3 Sheets-Sheet 5 BYQRW Patented Sept. 5, 1933 warren sra'rss r 1,925,160 ELECTRICAL FILTEBING APPARATUS Leland D. Whitelock, Wilkinsburg, Pa., assignor to The Union Switch & Signal Company, Swissvalc, Pa, a corporation of Pennsylvania Application October 17, 1931. Serial No. 569,477

9 Claims. (o1. 17s 44) My present invention relates to a new band pass filter of the feed-back or anti-reed back type and comprises a new circuit of high selectivity used may be arranged for example to respond to an input carrier wave of a predetermined frequency and in conjunction with a detector to transmit signals corresponding to a predetermined frequency only of the modulation current. Various other adaptations of the improved circuit will become apparent as the description proceeds. I

For a better understanding of the invention, reference may be had to the accompanying draw: ings, of which Fig. 1 illustrates a circuit adapted to pass and amplify a band of frequencies. Fig. 2 illustrates a modification of the circuit of Fig. 1 for passing high frequencies. Fig. 3 illustrates a band pass filter circuit including a Wheatstone bridge network for increasing the selectivity of. the circuit. Fig. 4 illustrates a modification of the circuit of Fig. l for eliminating a band of frequencies. Fig. 5 illustrates a plurality of circuits of the type of Fig. 1 combined in cascade and arranged for the transmission and amplification of modulation current of a predetermined fre quency selected from a carrier wave of prede termined frequency, automatic volumecontrol of vthe output being also included. Fig. 6 illustrates diagrammatically an alternative form of automatic volume control for the type of circuit illustrated in Fig. 5.

In Fig.1 the filter circuit includes an input transformer 1 connected across the input terminals 2, an output transformer 3 connected to output terminals 4, and ail-amplifier 5. The secondary of transformer 1 is connected in series with the primary winding of a step-up transformer 6 coupled to tube 5 and with the secondary of a feed-back transformer 7 the primary of which is in series with the primary of the output transformer 3 in the plate circuit of .tube 5. A variable capacity 8 is connected across the secondary of transformer 6 to form therewith a tuned input circuit for tube 5. A series tuned circuit, comprising an inductor 9 and variable capacity 10 is shunted across the primary of the feedback transformer '7 and a capacity 11 connected on one side intermediate elements 9 and 1 10 and grounded onthe other side serves totem: pensate for the distributed capacity and capacity! to ground of the various transformer windings; A battery 12, having its negative terminal grounded and its positive terminal connected to 6 the primary of output transformer 3, serves as a source of positive potential for the plate of tube 5; one side of the cathode of tube 5 being grounded through a grid-biasing resistance 13. The;

circuit including the secondaries of transformers 5 l and '7, and the primary of transforfner-fi, is grounded intermediate the respective windings or transformers 6 and 7. p

In the above-described circuit, theinput cir'} cuit of tube 5 is tuned to the frequency to be trans- 7o mitted, and the series tuned circuit, including the elements 9 and 10, is likewise tuned to this frequency. Capacity 11 is so chosen as to givemaximum sensitivity at the highest frequency tobe ATENT @FFHCE transmitted. The secondary winding of trans 75,

former '7 is so connected in the circuit as to have the instantaneous valueof the induced voltage therein due tofeed-backfrom the plate circuit" of tube 5 in a direction opposite to the instantane ous value of the voltage induced in the secondary 80.

of transformer 1 due to the reception of an incom ing signal. With this arrangement, when a signal of the frequency to be transmitted is impressed upon the input terminals 2, the voltage is stepped up by transformers 1 and 6 and amplified by tube 5. The plate current in flowing from the output transformer primary to tube 5 will take the path through inductance 9 and capacity "10 due to the relatively low impedance oiiered by this path at the resonant frequency compared to that" 0 through the primary of transformer'l. Thusfas no current, or substantially none; flows through the primary of transformer 7, no opposing electromotive force will be induced in the transformer circuit and the filter will operate as a'simple am? 1:

plifier. At other frequencies, the relatively high impedance oifered by thev elements 9 and 10 will" force current through the primary of transformer '7 and thereby cause an electromotive force to be induced in the secondary of transformer 'Twhich 911 operates to neutralize or balance outthe incoming signal. With location of the output transformer 3 between the battery 12 and feed-back trans former '7 as shown, the alternating current output of the filter system can bebrought to practically zero when the incoming frequency is sufficiently removed from the frequency of the series tuned: circuit because a path for the small alternating current through the'primary of transformer 7 required for neutralizing the signal is provided Q" through the capacity of transformer 7 to ground, and, in the specific circuit illustrated in Fig. 1, through the capacity 11 to ground. The provision of capacity 8 which tunes the secondary of transformer 6 by means of parallel resonance, is to cause the effective impedance in the grid circuit of tube 5 to increase greatly at resonant. frequency and thus to increase the signal output. At other frequencies the effective impedance in the grid circuit will be low, reducing the signal accordingly. An additional function of the variable capacity 8 is to make the circuit a tuned amplifier. Capacity 11, while not essential, is of value in making possible the same high degree of selectivity at high frequencies" as at low frequencies by balancing out the distributed capacities and capacities to groundof the various transformer windings at' high frequencies. For this purpose, capacity 11 will be extremely small. At low frequencies the capacites to be balanced out aretoo, small to cause. an appreciable drop in the selectivity ofthecircuit. 1 If the filter of. Fig.1 is modified, as shown in Fig-2,,an-efiicient high pass filter is obtained In Fig. 2 the series. tuned circuit of Fig- 1. shunting the. primary of. transformer '7 is replaced by a filter. section comprising capacities 14 and 15 conneetedinseries across the primary of transformer '1. and. an. inductance l6. grounded at one end and connected. at-the. other end intermediate the capacities 14. and 15. With this arrangement, the filter section offers relatively high impedance at low frequencies and forces the plate current through. the primary of transformer 7 with the reaultthatthe incoming signal is balanced out by the-opposing electromotive force fedback into the circuitof. tube. 5. Athigh frequencies, the relatively low impedance offered by the filter sectinn-causesthe alternating current component of the plate current. to be by-passed about the priwary of transformer 7 with consequent transmisoi the signal by the circuit. The values of capacities. l4. and. 15 and of inductance 16may be adriusted; to give the desired. cut-off point for the low frequencies.

Animportant modification of the circuit of Fig. lis shown in Fig. 3. In Fig. 3 the primary of transformer 'Z is bridgedacross a Wheatstone hridgenetwork. 17., in one arm of which is a series tuned circuit comprising an inductance 18 and capacity 19-, and in another arm of which isaparallel tuned circuit comprising an. inductanceQOsa-ndvariabl capacity 21 resistors 22' and 23 forming the other arms of the bridge. The

pnimary of output transformer 3v is connected to bridge. 1'?- intermediate resistor 22 and. the series tuned circuit and. the plate oftube 5 is connected. to bridge 17.- intermediate resistor: 2-3 and the. parallel. tuned. circuit. Resistor 22. is so chosen. as to be equal to the resistance of the series: tuned. circuitv at resonance, and resistor 23- is. so chosen as to be equal tothe resistancev 01 the parallel tuned circuit. at resonance.

operation of. the circuit of Fig. 3, when the signal has the frequency at which the two tuned circuits resonate, the bridge 17 will be imbalance 1nd,. therefore, substantially no currentwill flow through. the primary of transformer 'Z- At-any otherfrequency, the impedance of the series; tuned circuit. will increase, that. of the parallel tuned circuit will decrease, unbalancing thebzidge and causing energy to be fed. back into the; input circuit to neutralize the incoming. signah Thua. by means of. the bridge network, ad-

vantage may be taken of the change of impedance with frequency of both types of tuned circuit to make the filter very selective, the constants of the circuits may be readily adjusted, and the alternating current resistance of the primary of transformer 7 need not be as closely regulated as in the case of the circuits of either Fig. l or Fig. 2. Additional selectivity of the filter circuit may be obtained by tuning the grid circuit of tube 5 as in Fig. 1, if desired.

The circuit of Fig. 1 may be readily modified as shown in Fig. 4 so as to eliminate, rather than pass, a. selected band of frequencies. For this purpose the series tuned circuit of Fig. 1 shunting the primary of transformer 7 is replaced by a parallel tuned circuit comprising the inductance 24 and variable capacity 25. Thus, as the effective impedance of this parallel tuned circuit will be high at resonance, signals of such frequency will be choked by the counter electromotive force induced in the secondary of transformer '7 due to the current forced through the relatively low impedance path provided by the primary of transformer 7. At other frequencies, the parallel tuned circuit offers small impedance to the plate circuit, consequently, the impressed signal will encounter substantially no opposition to transmission.

In Fig. 5 a complete receiving system utilizing, circuits of the type of Fig. 1,,cnnectedin cascade, is illustrated. This system includes two high frequency stages with a buffer stage between to avoid loss of voltage gain, each high frequency stage being adapted to filter and amplify a band of frequencies, a detector stage, a low frequency stage arranged. to filter and amplify the low frequency output from the detector and a. rectifier for the output of the low frequency stage. The system also includes a measure of automatic volume control responsive to the output of the. system. and controlling the input to the first high frequency stage and tothe low frequency stage- The first high frequency stage. is coupled to. input. terminals 2. of the systemthroughthe. input transformer 1a. This stage. includesthe amplifier a,. transformers 6a and 7a, grid biasing battery 13a.- and. a series tuned circuitcomprising. theelements 9a and a shunting the primary of transformer 7a. The source of plate potential 12 isconnected through. the primary of output transformer 3a for this stage and through the tuned. circuit and primary of transformer 7a in. parallelwith the. plate of tube 5. As described in connection. with. Fig. 1, this stage will amplify and transmitonly frequencies differing slightly from that to which the circuit comprising elements 9a and 10av is tuned, other frequencies causing induction of counter currents in the input. circuit. of tube 5a.. A buffer stage including an amplifier 26 is connected between the output transformer 3a and the input transformer 11) for the second high frequency stage. ilar to the first high frequency stage and. to the circuit of Fig. 1, may have the seriestuned circuit. thereof so adjusted, if desired, as to cause this stagev to amplify and transmit a slightly different band of high frequencies, making the combination of the two stages pass a slightly wider band of frequencies than either alone. The output transformer SD for this second high frequency stage is coupled to any suitable detecting circuit, indicated diagrammatically at 27.

The output of the detecting circuit 27- is coupled to the input. transformer 10 of a low frequency filter stage having the series elements- 9c and 100 tuned. to the frequency of the modulation current This second stage, which is simto be transmitted. The output transformer of this low frequency stage is then coupled to any suitable rectifying circuit indicated diagrammatically at 28 and the output therefrom connected to the output terminals 4 of the system. The system above described will select and amplify a band of carrier wave frequencies, the particular frequencies transmitted depending upon the adjustments of the series tuned circuits of the two high frequency stages; will, after detection, transmit and amplify modulation currents of a prede termined frequency, depending upon the adjustment of the series tuned circuit of the low frequency filter stage. The direct current output from terminals 4 may be advantageously used for operating relays, or the like, to indicate the reception of carrier waves of a predetermined frequency having superposed thereon modulation current of a predetermined frequency.

In the particular system disclosed in Fig. 5,

means are provided for maintaining a substantially constant volume of current in the output circuit. These means comprise additional secondary windings 29 and 30 on transformers 1a and 10, respectively, each connected across the output terminals 4 and so arranged as to tend to magnetically saturate the cores of the respective transformers with increasing direct current in the output circuit and to thereby cut down their respective transformation ratio, and, accordingly, to cut down the strength of signal transmitted. A larger signal will increase the current through coils 29 and 30, thus increasing the magnetizing current and reducing the incoming signal. Conversely, a smaller signal will decrease the magnetizing force of coils 29 and 30 and thereby increase the strength of signal passed by transformers la and 10. A particularly advantageous feature of this volume control arrangement is the fact that a strong signal of proper carrier frequency but wrong modulation frequency will not operate the volume control; thus leaving the system in a sensitive condition for the reception of weaker signals of the proper carrier and modulation frequencies.

Another arrangement for automatic volume control of the circuit of Fig. 5 is illustrated in Fig. 6. In Fig. 6 the circuit of Fig. 5 is indicated diagrammatically, the various stages thereof being labeled and only such portions being indicated as are concerned with the volume control. In this arrangement the negative potential at the output of the rectifier is used to vary the negative bias on the tubes of both high frequency stages, and of the buffer and low frequency stages. For this purpose the high potential output side of the rectifier is grounded, and a conductor 31 connects the low potential output side through high resistance elements 32, 33, 34 and 35 with the grids of tubes 5a, 26, 5b and 50, respectively, and capacities 36, 37, 38 and 39 are inserted in the respective grid leads of these tubes. With this arrangement, as the output increases, the negative potential of conductor 31 is increased and thus the grid bias on all tubes is increased with corresponding reduction of the amplification factor of each tube. This method of volume control has all of the advantages discussed in connection with the volume control of Fig. 5 and has the further advantage that, no current being required for its operation, all energy is available for operation of output devices, such as relays.

It will be understood that the system of Figs. 5 and 6 may be varied widely. Any number of high frequency stages may be used, with or without buffer stages therebetween. Additional low frequency stages could be added if desired. The particular circuits used in the various stages could be varied and four or five element tubes could be used in place of the three-element tubes illustrated.

Circuits of the type illustrated in Figs. 2 and 3 could also be combined in cascade with or without volume control, if desired, to increase the selectivity of the filter in passing a band of fre-' quencies, and several circuits of the type illustrated in Fig. 4 could be similarly combined to eliminate more than one band of frequencies.

Various circuits and combinations of circuits embodying the inventive concept have now been described. Obviously, various changes could be made in the specific circuits illustrated without departing from the spirit of the invention and various features of one circuit could be combined with features of another circuit.

The following is claimed:

1. An electrical filter comprising in combination, a vacuum tube amplifier, an input circuit coupled thereto, a source of alternating electromotive force for said input circuit, a feed-back transformer having its primary winding in the plate circuit of said amplifier and its secondary winding so connected in said input circuit as to have the instantaneous value of the electromotive force induced therein in a direction opposite to hat of said source, a source of potential for the 1 plate of said amplifier, an output device connected in the plate circuit of said amplifier between said last mentioned source and the primary of said feed-back transformer and a network having an impedance varying with the fre- 1 quency of the current in the, plate circuit of said amplifier, said network being so associatedwith the primary of said feed-back transformer as to vary the current therethrough as the frequency of the current transmitted by said amplifier 1 varies.

2. An electrical filter according to claim 1 wherein said input circuit is coupled to said amplifier by a transformer and wherein a variable capacity is shunted across the secondary 1 thereof to permit tuning of the grid circuit of the amplifier.

3. An electrical filter according to claim 1 wherein the primary of said feed-back transformer is connected in series between the plate 1 of said amplifier and said device and said network is connected in parallel with the primary of said feed-back transformer whereby the distribu tion of the plate current between the primary of the feed-back transformer and said network varies with the frequency of the voltage impressed upon said input circuit.

4. An electrical filter according to claim 1 wherein said network comprises a circuit tuned to resonate at a predetermined frequency and wherein said tuned circuit and the primary of said. feed-back transformer are connected in parallel with each other and in series with said output device whereby the distribution of the plate current to said amplifier between the pril mary of said feed-back transformer and said tuned circuit varies as the frequency impressed upon said input circuit departs from that at which said tuned circuit resonates.

5. An electrical filter according to claim 1 wherein said network comprises a Wheatstone bridge, one arm of which is tuned for series resonance, a second arm of which is tuned to parallel resonance at the same frequency and the other arms of which are fixed impedances equal respectively to the impedances at resonant frequency of the first and second arms, and wherein the primary of said feed-back transformer has one end connected intermediate the fixed impedance arms and the other end connected intermediate the variable impedance arms; the plate of said amplifier and the output device being connected to the other terminals of the bridge whereby increasing plate current flows through the primary of the feed-back transformer as the bridge departs from balance due to a departure from resonance frequency of the voltage impressed upon the filter.

6. An electrical filter comprising in combination, an input circuit, a vacuum tube amplifier coupled thereto, a feed-back transformer having its secondary in the input circuit and one end of its primary connected to the plate of said amplifier, a source of potential for the plate of said amplifier, an output device connected between said source and the other end of the primary of the feed-back transformer, a series tuned circuit shunting the primary of said feed-back transformer, and a capacity connected to said tuned circuit for balancing the capacity to ground of said feed-back transformer, the negative terminal of said source, said capacity and the secondary of said feed-back transformer all being grounded.

7. An electrical system comprising in combination, a plurality of electrical filters connected in cascade, each of said filters including an input circuit, a vacuum tube amplifier coupled thereto, and a feed-back transformer having the primary thereof in the plate circuit of the amplifier and the secondary thereof so connected in the input circuit as to induce opposing electromotive forces therein, a source of potential for the amplifiers of all of said filters and an output transformer for each of said filters, the primary of each of said output transformers being connected between said source and the primary of the feed-back transformer of the associated filter, the output transformer of the first filter serving to transmit current to the input circuit of the suceeding filter, each of said filters including a tuned circuit so associated with the primary of the feed-back transformer of that filter as to increase the current there-through as the frequency impressed upon that filter departs from that to which the circuit thereof is tuned.

8. An electrical system according to claim 7 wherein the tuned circuit of at least one of said filters is tuned to resonate at a high frequency of the order of carrier wave frequencies, and the tuned circuit of at least one other of said filters is tuned to resonate at a low frequency of the order of modulation current frequencies and wh rein a detecting circuit is coupled between the output of the high frequency filter and the input of the low frequency filter whereby the system as a whole operates to transmit signals having predetermined carrier and modulation frequencies and to balance out those of a different carrier or modulation frequency.

9. An electrical system according to claim 7 including a rectifier connected to the last of said filters and wherein means responsive to the direct current output from said rectifier are provided for varying the volume of input to at least one of said filters in a direction tending to maintain constant volume of output from the system.

LELAND D. WHITELOCK. 

